What is the obsession with class balance? Why should a magic-user and fighter or rouge and cleric all be comparable in power at the same levels? Isn't balance a subjective quality that can shift based on play conditions?
Certainly balance
shifts based on play conditions, but that doesn't mean there's no central tendency.
You should probably ask, what does
level even mean? If we were designing a pure simulation, we might ask, how much does a fighter learn about fighting by overcoming a dozen foes in a dungeon? And how much does a magic-user learn about magic by doing the same? And
that would determine their skill and power progression from level to level.
Not only is that difficult to assess, but it makes for a lackluster game. What we really want is for the
players of the game to all feel
engaged while playing the game, and that means playing characters who can
contribute meaningfully in the game.
So, we define similarly useful characters of different types as characters of the same
level, but of different
classes, and we try to keep the party composed of characters of the same level, so everyone can contribute.
There are other ways to keep everyone engaged, of course, but making everyone similarly powerful and thus similarly useful is one tried and tested method. Other options might include boosting the usefulness of less powerful characters, by giving them more intangible
fate points (or whatever), giving the less powerful characters more powerful equipment (like a low-level halfling rogue with a
ring of invisibility), giving the less powerful characters a more central role in the story (heir to the throne, target of the Dark Lord, love interest of someone important), or having players control a portfolio of characters.